1. Field of the Invention
The present invention relates generally to a system for controlling the thickness and distribution of a coating applied to a moving substrate. More particularly, the invention relates to novel and improved apparatus for controlling the thickness and distribution of zinc coating applied to a steel substrate in a "hot dip" galvanizing process.
2. Prior Art
In a "hot dip" coating process, a moving substrate such as steel is coated with a corrosion resistant material such as zinc by feeding the substrate through a coating bath. The substrate emerges from the bath along a generally vertical feed path with molten coating material deposited on its surfaces.
The profile of the coating deposited on the substrate must be controlled to assure a substantially uniform coating on the resulting product. Profile control is also important to prevent wasteful deposition of excessively thick coatings and to assure that the coated substrate will perform in a consistent and desired manner in such handling processes as coiling, stacking, and shipment, and in such fabrication processes as die forming and welding.
There are two aspects of profile control which require careful control. The first aspect is coating thickness which is expressed in the art as coating "weight." Coating thickness is specified in ounces per square foot when a coated product is ordered. The second aspect is coating distribution, which is often preferably nonuniform in cross-section across the width of a substrate. For example, distribution of coating with thinner coating thicknesses in marginal portions is often desirable.
The thickness of the coating deposited on the substrate as the substrate travels through and emerges from the coating bath is dependent on a number of factors including the line speed of the moving substrate. While most of the coating thickness determination factor can be held relatively constant to minimize their influence on coating thickness, line speed remains a variable. Line speed is preferably maintained at a relatively high velocity during the majority of a coating run, but must be significantly reduced from time to time to permit the welding of a new source of substrate to the depleting source of substrate being coated. When line speed decreases, the thickness of coating material removed from the coating bath by the moving substrate decreases correspondingly. When line speed increases, a corresponding increase in deposited coating results.
The distribution of coating deposited on a substrate as the substrate emerges from the coating bath ordinarily varies considerably from that which is desired. The uniformity of the deposited coating improves due to surface tension forces and the tendency of the coating to flow as the substrate moves upwardly along a path of travel from the coating bath.
Coating thickness and distribution can be modified after a coated substrate has emerged from the coating bath. Various coating control systems have been proposed for use near a coating bath to modify the profile of the coating deposited on a substrate. Proposed control systems have included such devices as rolls which engage the coated substrate, and fluid devices which do not engage the substrate but rather direct controlled flows of pressurized fluid toward the coated substrate.
A problem with proposed coating control systems has been their inability to maintain a predetermined coating profile during changes in line speed of the moving substrate. While some control systems have been proposed to assure that the amount of coating screeded from the substrate decreases with a line speed decrease and increases with a line speed increase, substantial variations in coating profile result during line speed changes.
Similar problems are encountered with proposed coating control devices in obtaining the same coating profile on substrates of different thicknesses. Relatively thick substrates are, in many systems, run at lower line speeds than relative thin substrates because thicker substrates require greater coating bath immersion time to bring them to a proper temperature for coating deposit. While the same coating profile may be desired on both thick and thin substrates, these substrates may well be run at different ranges of line speeds. The coating control devices should, accordingly, not only be operable to maintain coating profile when line speed changes within a given range for a given substrate, but should also be operable to provide the same given coating profile within different line speed operating ranges. Proposed coating control systems have not provided this desired degree of versatility.
A further problem encountered in the coating of substrates is that some substrates require heavier or thicker coating applications than others. Where the substrate will be subjected to a highly corrosive environment, an application 21/2 ounces or more of zinc coating per square foot is often sought. Other substrates may require an application of only 0.25 ounce or less of zinc per square foot. Controlling coating thickness accurately within such a wide range of operation has been difficult with prior coating control systems.
Still another problem encountered in the coating of substrates is that different coating distributions are preferred depending on the use which is to be made of the coated substrate. Proposed coating control devices have been difficult to adjust to obtain any substantial change in coating distribution.
Where the substrate being coated is to be coiled for storage and shipment, it is advantageous to provide a slightly convex distribution of coating on the substrate with the coating at its thickest near the center and progressively thinner further from the center. A convex coating surface mitigates the tendency of a substrate to "spool" during coiling and helps minimize damage which may result in peripheral regions due to stretching and deformation during coiling.
Where the substrate being coated is cut into relatively short lengths and stacked rather than being coiled a more nearly uniformly thick coating is desired. In order to permit the coating of coilable and cut-to-length substrates on the same processing line, it is desirable to provide a capability to alter distributions from uniform thickness to any of a range of desired convex configurations. Previously proposed coating control devices have not given this desired versatility.
While it would be desirable to provide a coating control system which can be set at the beginning of a coating run to provide a particular coating profile, proposed coating systems have not been sufficiently reliable in operation to provide the desired profile.
Accordingly, coating thickness and distribution is normally monitored in present-day practice by beta or gamma ray, back-scatter coating gauges.
These back-scatter gauges scan the width of a coated substrate and instantaneously provide data on coating thickness and distribution. The data from such gauges can be fed automatically to computer systems which operate the control devices to effect any needed correction in coating thickness and distribution. A problem with such systems is that the beta or gamma ray gauges are necessarily located as much as several hundred feet downstream from the coating control devices. Where such gauges are relied on as the primary source of data to operate the coating control devices, a substantial amount of improperly coated substrate can be produced before coating deficiencies are detected, compensations are made, and further data is received indicating what, if any, further corrections need be made. For this reason a substantial quantity of material may be wasted, especially at the beginning of a coating run.
Of the various types of proposed coating control devices, pressurized fluid-emitting devices have been found to most successfully control coating thickness and distribution. Pressurized fluid devices are preferred to rolls for a number of reasons including the fact that fluid control devices permit the use of higher line speeds, thinner coatings, and thinner gauge substrates than are possible with rolls. A smoother finish, free from marks and damage occasioned by the use of rolls, is attainable with fluid control devices.
A number of pressurized fluid mediums have been used in such devices with superheated steam and pressurized air being the most common. Pressurized air has been found to be preferably to steam for a number of reasons. These include:
a. Where steam is used, narrower nozzle openings are required. Any oxide scale that breaks loose in steam supply conduits can plug portions of the narrow nozzle openings, causing imperfections in coated substrate. PA1 b. Steam nozzles are found to generate substantially higher noise levels than pressurized air nozzles. PA1 c. The energy required to heat steam makes its use less economical than air. PA1 d. The attendant corrosion of surrounding equipment and buildings encountered with the use of steam.
The nozzles used to direct pressurized fluids onto a coated substrate as it emerges from a coating bath are known in the art as "air knives." An air knife typically includes a pair of elongated lips which are spaced to define a narrow elongated discharge opening. A plenum chamber or "header" is provided upstream from the discharge opening and extends the full length of the discharge opening. A convergent throat or transition section is provided between the plenum chamber and the lips to duct pressurized fluid from the plenum chamber to the discharge opening. A pressure distribution device such as a small mesh screen is ordinarily used in the throat section to assure that a laminar flow of equally pressurized air is delivered to and discharged from all portions of the discharge opening.
Two oppositely oriented air knives are used to direct pressurized fluid toward both faces of a substrate shortly after it exits from a coating bath. The position of the air knives is critical to the successful control of coating thickness and distribution. Proper positioning for a particular substrate, coating material, line speed and desired coating profile is ascertained through careful experiementation. In general, the knives are positioned between about 8 to 30 inches above the surface of the coating bath with their longitudinal axes horizontal. They are also positioned to discharge fluid toward the generally vertical path of the substrate. The knife lips are usually spaced about 1/2 to 21/2 inches from the moving substrate.
The knives may be inclined to discharge fluid upwardly or downwardly toward the substrate at angles between about 20.degree. above horizontal to about 45.degree. below horizontal. Angles of inclination of between about 10.degree. above horizontal to about 30.degree. below horizontal are found to be most useful, with angles of between about 5.degree. to 15.degree. below horizontal being preferred for galvanizing operations.
Curved air knives, i.e., air knives having curved lips, have been proposed to achieve a convex coating profile. The radius of curvature for such knives is typically about 13 feet. Where the curvature of such knives extends in a vertical plane, the actual radii of curvature of the top and bottom knives differ slightly to achieve a uniform height orifice opening along the full length of the knife lips.
In operation, the curved air knives are positioned such that the distance between the substrate and the knife lips is greatest at the center of the knife lips. The variation in distance from the substrate which is achieved by the curvature of the knife facilitates the formation of a coating of convex cross-section on the substrate.
Curved air knives discharge fluid at pressures which are substantially uniform along the length of the nozzle lips. Since the fluid discharged from central portions of the nozzle lips must travel a greater distance to reach the coated substrate than fluid discharged from end regions of the nozzle lips, the centrally discharged fluid has a lesser pressure than does the end-discharged fluid when the fluid reaches the substrate. Curved air knives may therefore be said to utilize a differential in pressure between the center and the edges of a substrate to provide a convex coating profile on the substrate.
Since a curved air knife designed for use on wide substrates will not afford desired pressure differential when used on relatively narrow substrates, knife changes are necessary between production runs. One proposal for knife changing suggests knives of a variety of lengths mounted at circumferentially spaced locations about a rotatable turret. When substrate width is changed, the proper length air knife is rotated into operating position. The escape of pressurized fluid from air knives not in use is prevented by removing their pressure distribution screens and substituting impervious baffle plates.
While curved air knives have been found to operate satisfactorily in the production of convex coating profiles, these knives have a number of disadvantages. A significant drawback of curved air knives is their very expensive fabrication cost. Accurately machining a 13-foot radius of curvature along surfaces that extend for several feet is expensive. Moreover, the upper and lower lips and their associated mounting parts are not interchangeable and require different machining operations to fabricate. A single set of curved air knife lips, at present-day material and labor prices, is found to cost about $17,000.
Curved air knives are not easily adjusted for use with a wide range of substrates, coating materials, line speeds and desired coating profiles. Uniform coating distributions having no convexity or concavity in their profiles are almost impossible to achieve using curved air knives.
When the inclination of a curved knife is adjusted to accommodate a new substrate or coating material, the distance from the center of the curved knife lips to the substrate changes even through the distance from ends of the knife lips to the substrate may be held constant. To state this problem another way, adjustment in one knife setting influences adjustments in other knife settings to a far greater degree in curved knives than in straight knives, whereby some compromises in desired adjustment of curved knives are frequently required.
The supply of pressurized fluid to air knives has previously been regulated by adjustable louvers positioned in the intake of a blower to regulate the blower's output. Pressure transducers have been provided downstream from the blower to sense supply pressure. Tachometer sensors have been provided on one or more of the rolls which feed the substrate to sense line speed. Signals from the pressure transducers and from the tachometer sensors have been fed to a computer system which generates an output signal to open or close the blower intake louvers as required to maintain desired supply pressures at various line speeds. Such a system of transducers, sensors, and computer controlled louvers is complex and expensive to build, install and maintain.